1. Field of the Invention
This invention relates to a thermosetting melamine-modified phenolic laminating resin having excellent penetration and rapid cure suitable for continuously making high-grade laminates at high speed and relatively low pressure.
2. Description of the Prior Art
Conventional decorative laminates typically are produced in a batchwise procedure by curing a plurality of thermosetting resin-impregnated fibrous substrate layers in a press at high pressures (e.g., 800-1500 psi) and elevated temperatures (e.g., 250.degree.-350.degree. F.). The laminates generally consist of three elements: a core section consisting of several sheets of an inexpensive substrate, e.g., kraft paper, saturated or impregnated with a resole-type phenolic resin, a print sheet of alpha cellulose saturated with an expensive amino resin, e.g., a melamine resin and a translucent overlay of rayon or alpha cellulose, also saturated with an expensive amino resin.
When preparing the laminate using this conventional process, the paper substrate first is passed through a dip tank generally filled with a solution of the laminating resin. Resin penetration of the substrate is substantially accomplished during immersion in the tank. Excess resin solution generally is removed from the substrate by opposed scraper bars or blades as the substrate leaves the dip tank. Residual penetration must be completed as the substrate is thereafter passed to the drying zone.
The drying zone generally comprises a long oven where the solvent for the resin is evaporated (down to about 5-10% by weight) and the resin is substantially advanced to a substantially non-tacky, infusible stage (B-Stage). The paper impregnated with the advanced resin is removed from the drying oven and then is cut to size, stacked and consolidated in the hot, high pressure press. Generally, the laminate is cured in the high pressure press for about 15 to 60 minutes. The laminate then is cooled, at pressure, to room temperature to ensure a flat, dimensionally stable panel and prevent blistering.
Because the laminate is consolidated in a batchwise fashion, the conventional high pressure laminating procedure is rather slow. This necessitates substantial capital investment in order to maintain high production rates, and consequently results in high production costs for the finished laminate.
In an attempt to improve the economics of the conventional laminating process, a continuous laminating procedure has recently been developed. In this process, a plurality of the fibrous, resin-impregnated (B-stage) substrate layers are consolidated at an elevated temperature by passing them between the nips of serially arranged pressure rolls. The cured and consolidated sheets then may be cut to the desired size at any time prior to use.
Because of the continuous operation, high production rates are possible. In this process, penetration and B-staging of continuous sheets and subsequent curing (consolidation) of a plurality of such sheets all can typically be completed in a more or less continuous manner in less than about 5 minutes. Consolidation pressures in this arrangement are normally limited, however, to below about 750 psi, e.g., about 200 psi, and therefore impose stringent requirements on the nature of the laminating resin. Not only must the resin quickly penetrate the substrate and flow under a relatively low consolidation pressure, but the resin also must subsequently cure in an extremely short time to yield a product with properties at least comparable to conventional laminates. Additionally, in some applications, to be commercially acceptable, the laminating resin also must exhibit a stability which allows the resin to be stored for a period of time prior to use, without undesirable advancement of the molecular weight and corresponding viscosity increase and reduction in penetration rate.
It is, therefore, a principal object of the present invention to provide a laminating resin that satisfies the stringent requirements of the continuous laminating process.
It is also an object of this invention to provide a phenolic laminating resin that has been modified to improve its cure rate.
It is a further object of this invention to provide a modified phenolic laminating resin that exhibits excellent flow under relatively low pressures.
It is another object of the present invention to provide a rapid curing, melamine modified phenolic laminating resin that exhibits a penetration suitable for the continuous lamination process.